1. Field of the Invention
The present invention generally relates to driver circuits, and, more particularly to voltage-mode driver circuits. Even more particularly, the invention concerns a voltage-mode driver circuit adapted for driving complex impedance loads, i.e., loads having a transfer (admittance) function with one or more poles or zeros, such that the load current does not continuously reflect the applied voltage.
2. Description of the Related Art
An example of a complex impedance load is a motor, which is an inductive load. Motors are used in a wide variety of applications.
For example, in modern high-density disk drive storage systems, such as hard disk drives (HDDs), a voice coil motor (VCM) controls the position of a read/write head with respect to a disk surface. The VCM is inserted in a closed control loop. Exploiting data recorded at various locations on the disk surface (such as track numbers univocally identifying each recording track, and sinusoidal burst signals staggered in position between adjacent tracks), information about the head position is obtained, and a head position error signal is calculated and fed to an electromechanical servo position system.
The servo position system includes a current-mode driver, controlled by a servo positioning system controller, which delivers a controlled current to the VCM. The current-mode driver comprises a high-power linear amplifier and a shunt resistor for measuring the actual current delivered to the VCM. The difference between the measured current and a reference current provided by the servo controller is amplified by the linear amplifier.
The control loop has a relatively large closed-loop bandwidth, typically of 50 KHz; this allows designing the servo controller without having to take into account the dynamics of the current-mode driver: roughly speaking, the designer of the servo controller can consider a static transfer function between the reference current and the actual current delivered to the VCM.
Current-mode drivers for driving VCMs have proven to be very reliable and to enable quite precise a control of the read/write head position, even in modern high-density hard disk drives. In particular, an advantage of using a current-mode driver is that the current flowing through the VCM does not depend on the VCM electrical impedance, nor on the electro-motive force generated by the rotational speed.
However, current-mode drivers are rather costly, both in terms of dissipated power and in terms of components cost. The power dissipated is high due to both the linear amplifier and the shunt resistor. Linear power amplifiers require a large silicon die area, and additional components, such as the shunt resistor, are required.
In order to drastically reduce the costs, the measurement of the current delivered to the VCM should be dispensed with. This means eliminating the current control loop.
An apparently readily available solution to the problem is replacing the current-mode driver with a voltage-mode driver. However, this solution has not been pursued so far, mainly because, differently from the current-mode driver, the performance of a voltage-mode driver is heavily affected by the VCM dynamics. This would require completely redesigning the servo controller, something which makes manufacturers of disk drive systems very reluctant.